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And in Illinois, three young people have been hospitalized for severe breathing problems after vaping, the state Department of Public Health announced Friday. "The names and types of vaping products, as well as where they were obtained, are still being investigated," the department said.

'Some even needed assistance to breathe'

Thomas Haupt, a respiratory disease epidemiologist with Wisconsin's Department of Health Services, said Friday that his state's cases were among young people, "otherwise normally healthy, and they were coming in with severe respiratory illnesses, and in some cases, they actually had to go to the intensive care unit and were placed on ventilators."

The lung disease looked like it was caused by an infection, "but every test has come back completely negative," Haupt told CNN. Regarding any links between the cases, "the only thing at this point is vaping, but we don't know what they vaped, where they got their vaping liquids, all this needs to be determined at this point."

Chuck Warzecha, a deputy administrator at Wisconsin's health department, said that "all of them were hospitalized with shortness of breath, fatigue and chest pain. Some even needed assistance to breathe." While the patients are "generally improving," it is unclear what long-term effects they might face, he said.

The majority of Wisconsin's cases were in the southeastern part of the state, said Haupt. In Illinois, the state's three hospitalized patients were from the northeastern part of the state, which borders Wisconsin. It's not yet known whether the cases in both states are linked to a common source, such as a vape product or e-liquid.

Dr. Ngozi Ezike, director of the Illinois Department of Public Health, told CNN that her team has "talked to people from Wisconsin" and is gathering data and running tests. "We have a lot of unanswered questions at this time," she said.

As in Wisconsin, the Illinois cases included coughing, shortness of breath, fatigue and sometimes chest pain that worsened over time, according to Ezike. The cases also occurred in people with "no known lung problems or previously diagnosed pulmonary issues," she said.

'Potentially toxic substances' in vapes, committee says

There were questions about the safety of vaping even before the recent hospitalizations. The American Lung Association, for example, says it is "very troubled by the evolving evidence about the impact of e-cigarettes on the lungs."

The organization points to research showing that key vape ingredients may harm cells or contain "dangerous chemicals" that can "cause lung disease, as well as cardiovascular (heart) disease."

An expert committee of the National Academies of Sciences, Engineering and Medicine reported last year that there is "conclusive evidence that in addition to nicotine, most e-cigarette products contain and emit numerous potentially toxic substances."

Scientists are unsure how those substances may contribute to lung disease, if at all. But the committee said there is "moderate evidence for increased cough and wheeze in adolescents who use e-cigarettes" and tied vaping to "an increase in asthma exacerbations."

'Who knows how many more cases we're going to get'

Gregory Conley, president of the American Vaping Association, said that "unless the Department is withholding information, what we know today does not justify discouraging adult smokers from using vaping products as an alternative to cigarettes."

It is "absolutely shameful that health departments are blaming vaping generally when the facts point to street-bought THC cartridges as being the likely cause of these hospitalizations," he said.

He pointed to local news reports that described one hospitalized Wisconsin patient as having purchased THC oil on the street before his breathing issues began, according to his brother.

But health officials have yet to identify any common cause across the incidents, and Haupt, the Wisconsin epidemiologist, said "we haven't indicated THC as a problem yet."

He said THC, marijuana's key psychoactive ingredient, "is part of our questionnaire that we're looking into to really see if there is indeed use of THC and how widespread it is amongst these cases."

Collecting information has been difficult, though, because "we're dealing with minors, so they aren't always the most honest when they deal with these things," said Haupt.

He added that the state has been in contact with the US Centers for Disease Control and Prevention and "the FDA has called as well." But this is the first time the state has seen a cluster of lung disease associated with vaping, Haupt said, and "who knows how many more cases we're going to get."